The present invention relates to a solid insulator which may be conveniently used in combination with a water-soluble insulating varnish, and to an electric equipment coil using the same. The present invention further relates to a method for insulation treatment of an electric equipment coil.
As a solid insulator which may be used in combination with a water-soluble insulating varnish containing as a solvent water or dimethyl ethanolamine, a polyimide film is known having excellent resistances to hydrolysis, chemicals and solvents, or a combination thereof with another material. However, such an insulator is expensive and may not be used in practice.
In view of this problem, it has been recently proposed to use as such an insulator a heat-resistant aramid paper having a thickness of 0.075 to 0.6 mm or a combination thereof with another material. An insulator of this type is inexpensive. However, if such an insulator is actually used as a slot insulator of an iron rotor core, and heat-resistant aramid papers are adhered to both surfaces of an inexpensive polyester film, the overall thickness of the insulator becomes great. This results in a decrease in space factor within the slots, and in complex assembly operation due to poor slip characteristics of the heat-resistant aramid paper.
If a polyester film is used alone as the insulator, the above problem may be solved. However, the following new problems are encountered. When the polyester film is used alone, the solvent component such as water or dimethyl ethanolamine contained in the varnish causes a chemical reaction such as hydrolysis or crystallization depending upon the combination of drying temperature and time (e.g., 150.degree. C. or higher for 4 hours or longer) after varnish treatment. When such a chemical reaction occurs, the tensile strength retention is decreased to about 50%, which results in a decrease in flexibility, and in degradation of the characteristics of the polyester film.
If a polyester wire which is generally used for a winding is combined with a water-soluble insulating varnish, the polyester enamel film is chemically modified under the heat of drying, resulting in degradation of dielectric breakdown strength.